1. Field of the Invention
The present invention relates to wafer handling systems and more particularly to an apparatus for transferring wafers into and out of processing, wherein the apparatus automatically calibrates the necessary positions of its related parts.
2. Description of the Prior Art
In fabricating semiconductors, silicon wafers are often held in a cassette and then moved to various pre-programmed processing locations by a robotic handling system. The latter typically includes a mechanism with degrees of freedom in radial (R), angular (.theta.) and vertical (Z) directions and has a robot arm with a vacuum or edge-gripping wand. The robot must be able to pick up wafers from a storage cassette and then transfer them to a designated station or a plurality of stations where the wafer will undergo a process such as heating or alignment. In order to perform these actions, the robot must have precise knowledge of the R, .theta. and Z positions of where the wafer is to be placed at all cassette and station locations. A robot control system must include the knowledge required for positioning the robot arm with gripped wafer precisely within a cassette or process station for each robot function.
In a typical wafer handling layout the general geometry of both the robot and the various process stations such as the cassette stand are known, and the dimensional relationships between the robot and each station are known within nominal tolerances (e.g.=0.10 inches), available from CAD drawings or manual measurements. When in use, however, the robot must be controlled to move wafers more precisely in order to assure that the robot system operates properly without damaging any system component or the wafer being handled.
In order to assure the close tolerances required for the necessary precision, the controller of the robotic element must be reprogrammed or "re-taught" new location data whenever a component is changed, or upon initial setup or when restarted. The term "teach" or "teaching" will be used to describe the process of gathering and entering component/structural location data into the system controller. Due to the need to minimize contaminants in the semiconductor processing environment, most robotic systems are installed in enclosures for control of the atmosphere. In prior art systems, it is generally necessary for a technician to enter the enclosure to position the robot while performing the teaching/calibration operations. These entries can contaminate the clean enclosure. In addition, the cramped, confined enclosure with moving robot parts presents a significant safety problem for the technician. This manual and awkward process is also time consuming and costly, and an inherently subjective process that relies upon the judgment and skill of the technician.
For example, using conventional controls, a robot is installed and taught by jogging the robot around and, at each process station, the wafer placement locations are recorded with a teach pendant. Besides consuming many hours, this manual procedure introduces subjectivity and thus a significant possibility for errors. This creates a problem of reproducibility. Whenever a wafer cassette is not perfectly positioned within specification or a machine component wears, settles or malfunctions and requires replacement, the robot must be re-taught because it cannot automatically adapt to such variations. If the robot is not re-taught properly within close tolerances, serious damage or loss of expensive wafers can result.
It is clear from the above description of the prior art that an improved system for handling wafers is needed to eliminate the requirement of an operator entering the wafer handling enclosure environment for calibration/teaching operations.